In my case the screen above shows all devices and their partitions: ada0 da0 da1 (hardrive and two USB thumb drives respectively). da0 contains the freshly created partition table for the FreeBSD install, just as it should.

Confirmation Your changes will now be written to disk. If you have chosen to overwrite existing data, it will be PERMANENTLY ERASED. Are you sure you want to commit your changes?

What exactly will happen if I click/choose <Commit>? The previous screen displayed all partition tables for all devices, will the installer touch ada0 and da1 as well or just da0 (which is what I want). It's the way the information is laid out by the installer which makes me hesitant to proceed.

There is a stage where you select the device node of the device you want to install to, create a GPT or MBR, etc and then create a FreeBSD partition and set up your slices from there. At that stage you should not be seeing the other devices....?

I haven't tried to repeat the process since then, as I managed to install OpenBSD on the machine in the meantime (at least in BIOS legacy mode). FreeBSD was just a detour. My premise was to avoid as much as possible any risks that could damage the OSX install on my macbook. I was attempting a USB to USB install: installer booted from USB pendrive with a second USB pendrive as the installation target.

What you describe sounds like the steps I outlined in my bullet-list above. The problem was, if I remember correctly, that immediately before the confirmation warning, there was a screen that showed all(!) devices and not just the one I had selected as an installation target. That's why I bailed out. I was afraid of any changes to the machine's main harddrive.

I probably should add that I have no FreeBSD experience whatsoever. Other than configuring an ad-hoc iscsi server by booting the FreeBSD installer from USB into the live system, no installation required. Which in itself is pretty cool.

I'm not sure about installing on macbooks, having zero experience with that hardware.

Installers for most of the *BSD operating systems are comprehensive and robust, but not exactly forgiving, or designed around "dual boot" scenarios, so bailing out was probably wise. I've had a few accidents with the OpenBSD (my OS of choice) installer in the past (ultimately my fault of course).